1. Field of the Invention
The present invention relates to a loop gain setting method for for automatically setting the loop gain of the focus servo loop and tracking servo loop of a disc player.
2. Description of Background Information
In optical disc players for playing an information recording disc (simply referred to as disc hereinafter) such as a video disc or a digital audio disc, it is necessary to use a focus servo system for converging the light beam onto the surface of the disc to form a light spot for reading information and a tracking servo system for moving the information reading point so that it follows the recording track accurately.
As the focus servo system, systems according to a so-called astigmatic method are known, which systems are arranged such that the light beam is converged, by using a cylindrical lens for example, as a focal line in horizontal direction at one of two points on the light path of the light beam which are apart from each other, and also converged as a focal line in vertical direction at the other of the two points, a light sensor of a quadrant type is disposed at a middle position of the two points to generate an error signal, and a focus error signal is generated from four output signals of the quadrant type light sensor
As the tracking servo system, systems according to a so-called three-beam method is known, which systems are arranged such that three beams including a main beam for reading information and two sub-beams for detecting tracking errors disposed on both sides of the main beam are prepared and arranged so that a line passing centers of the three beams forms a predetermined off-set angle with respect to the direction of the track, and an error signal is generated from a difference between the amount of the two sub-beams reflected by the recording surface of the disc.
In these servo systems, the loop gain of each servo system can be varied for each disc to be played, as a result of a variation in the reflectivity of discs, a variation of optical power of the pickup due to dirt or dust adhered to the pickup in association with its time change, or a variation in the sensitivity of the light detector of the pickup. This variation in the loop gain will adversely affect the operation of the servo loop to follow the focus error or the tracking error, so that the stability of the servo loop is reduced.